Stable reference apparatus



March 17, 1953 w. G. WING 2,631,455

STABLE REFERENCE APPARATUS Filed June 29, 194B INVENTOR W/LL/s G. WING Patented Mar. 17, v1953 UNITED STATES PATENT OFFICE Appiiatio' June 29, 194.61,u serial No; aso-seis The present invention relates to stable refer-"- ence apparatus, and more particularly, to apparatus employing gyroscopio" stability for providing an orientation reference.-

It is well known that a gyroscope embodying a universally suspended mass rotated' at high speed may be employed for establishing1 a stable reference with respect to two perpendicular' axes in space, and that twov gyroscopes may be employed for establishing.' al universal reference about three perpendicular axes. Ordinarily, agyroscope for a stable reference about two axes embodies a spinning rotor borne in a frame pivotally mounted in a gimbal ring-which latter isin tur-n pivotally mounted in a iixed supporatherotor spin axis and gimbal ring pivot axes being mutually perpendicular. Variations of orientation of the body pivotally supporting thefgiinbal ring ordinarily are denoted by relative movements between the'y gimbal ring' andA the rotore-bearing; frame-or relative movements betweenl theg'imbal ring and the' body, according tof the axis'pabout which the body turns. Sensitive pick-ons have been` provided at the gimbal` pivot/axesA for transmitting orientation data. signals to controldevices, er g.,

to an auto-piloter to algun-directing apparatus,

for control ofsuch:devicesinacco`rdance with the changes of orientation of 'the body:

Although one gyrosc'opef is usable for` orienta;r` tion reference about two axes, it is subject to minute changes-o. orientation due todisturbing forcesyin such a way that-errors- 'are-mtroducedis exerted upon thegyroscope about the r'st axisV througlithe frictionat dragoftthefgimbalbearings- While such. a; torque'f .infoducesLsubstantiallyv no rotation' of the gyros'cope spin'faxisaboutfthelrst horizontal axis, it, produces aminute preces'sional'f rotation of' thef spin-axis: about the second liori= zontal axisf errolrfis aoeordin'gly' introduced intoA the representing; craft 'attitude with respecttdthesecondfhorizo tal axis; Similarly', rotation .of thezcraft' aboutflthei secondxhorizontal axis induces the; gl'yrlof tof change its orientation. aboutthe iirsti' axis; r and: hence'. produces? error in the signal representingi craft" attitude' about the" first axis'.

Where: nignf rsf required. these ii Claims. (C1. 744534) errors cannot be tolerated. Moreover, ifthe sensitive pick-'offs exert reactionary forces upon the gyroscope, such forces' are added to the bearing frictional drag forcesr magnifying the data signal In accordance with an invention by Robert F. Hays, disclosed and claimed in aU. Si Patent No. 2,591,697, issuedApr-il 8,1952, andI entitled Stable Reference Apparatus. a" movably supported frame to be stabilized about predetermined axes in space is provided with aff-gyroscope arranged for stabilization about each' suchaxis. The gyroscope itself affords stiin'ess about the axis for which it is intended to afford stabilization, firmly resisting rotation aboutl such axis but responding from gimbal error, and withextremely high rigidity in spaceof the movable common supportingY frame for the three gyrosoopes-L Itis an objectof theI present invention to .prof

vide an improved stable reference system. n

More specically,it is an important object vofV the present invention to provide a gyrosoopic sta-A ble reference system with improved orientationV control othe gyroscope combining directness with extreme simplicity.-

Another object of the7 present inventionis to provide a gy'roscopicy stabilization system affording extremely high accuracy and capable of providing pick-on signals of appreciable strength without introduction of objectionable reaction error in theorientation of the g-yroscopic reference system, while providing ideally simplified gyroscope orientation control. e In accordance withVA an important `feature of the present inventionfagyroscopein the above-` earths magnetic field, the action of the force' iield' upon thegyroscopebeing so directed as tol produce prece'ssion of they gyroscope in the direction'`Av tov restore the desired orientation ofl thev frame-f Where" the: frame is' providedy with onegyroscope arranged to maintain the frame in a fixed azimuthal orientation and two further gyroscopes arranged to stabilize the frame about mutually perpendicular horizontal axes, the former gyroscope may be provided with a bar magnet arranged for responding to the earths magnetic force neld and the other gyroscopes may be arranged with their centers of gravity displaced from their normally vertical gimbal journal axes in the direction of downward peripheral movement of the rotors, for response to the earths gravitational force neld.

The invention in another of its-aspects relates to novel features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used for the said principal objects or in the said field.

A further object of the invention is to provide improved apparatus and ins-trumentalities embodying novel features and principles, adapted for use in realizing the above objects and also adapted for use in other fields.

The above objects and features will become more clearly apparent, and other objects will be recognized, from the following description of a preferred embodiment of the present invention, as illustrated in the drawing, wherein Fig. l illustrates a universal stabilization apparatus embodying the features of the present invention, and Fig. 2 illustrates details of an azimuthal transformation device included in the universal stabilization apparatus.

In Fig. 1 is shown a frame Il universally suspended for freedom about three mutually perpendicular axes, and provided with three gyroscopes I3, l and I1 for detection of and opposition to forces tending to disturb the orientation of the principal frame Il. Frame Ii is illustrated as pivotally supported for freedom about the vertical axis in a Cardan suspension element I9. Element I9 is illustrated as pivo-tally supported for freedom about a first horizontal axis 23-23 in a further Cardan suspension element 2|. pivotally supported, as bybearings installed in a craft, for freedom about a second normally horizontal axis 25445, perpendicular to axis 23-23.

Gyroscopes I3 and i5 are provided for affording stabilization of frame II about the mutually perpendicular horizontal axes, and gyroscope I? is arranged to stabilize frame II about the vertical axis. Gyroscope I3 embodies a rotor si supported for rotation about a horizontal axis in a rotor-bearing frame 29 which, in turn, is pivotally supported vabout a vertical giznbal journal axis in the principal frame II. The rotor-bearing frame 29, while diagrammatically illustrated as an open ring, may be a rotor casing completely enclosing the rotor 21. Motive means (not shown) are provided for driving or spinning the rotor 21 in high speed rotation. For-this purpose, an electric motor may be incorporated with a stator xed to frame 29 and a rotor attached to or forming part of gyroscope rotor 21. Such well-known gyroscope constructional features are similarly applicable to gyroscopes. i5 and I1.

Gyroscope I3 aifords rigidity to oppose rotation about the horizontal axis perpendicular to its rotor axis. With the relative orientations of frame II and element i9 as illustrated in Figi 1,- gyroscope I3 affords stabilization of frame merely about axis 23-23. Any. forces tending Cardan suspension element 2|, in turn, is

to change the orientation of frame I I about axis 2S- 23, e. g., bearing frictional forces transmitted through the bearings of the Cardan suspension system when the supporting body or craft revolves about the axis 2li-23, are opposed by the gyroscopic rigidity resulting from high-speed rotation of the rotor 21; but such forces result in such precession of the gyroscope |3 that the rotor-bearing frame 29 commences to revolve very gradually about the vertical girnbal journal axis relative to the principal frame II.

Sensitive pick-oil apparatus schematically indicated at 3| as an E pick-off, embodying a signal transformer unit having relatively movable core portions 33 and 35 provided with a neutrally positioned primary winding and series-opposed secondary windings, on the E legs, in accordance with the teachings of U. S. Patent 1,959,804 to Wittkuhns et al., is provided for supplying an output signal of phase and magnitude denoting the direction and extent of angular displacement developed between the rotor-bearing frame 29 and the principal frame II. The opposed secondary windings of the pick-off 3| are coupled, as through a transformation device 31 and an amplifier 39, to a suspension axis motor il coupled to suspension elements i9 Yand 2|. The device 31, the amplifier 3s and the motor 4| are so connected and arranged that the motor il exerts a torque in the directionv producing procession of gyroscope i3 to overcome the rela` tive displacement between the parts 33 and 35 of pick-off 3|, and thus to prevent prolonged exertion of any net disturbing force on the frame ll about the axis perpendicular to the spin axis of rotor 2l'.

The second gyroscope I5 similarly embodies a rotor i3 supported for rotation in a rotorbearing frame 45 which is journaled in a vertical gimbal journal axis in the principal frame II. A second sensitive pick-oil 41,' which also may be of the E type, is provided for supplying an output signal upon precessional departures of the second gyroscope l 5 from a predetermined orientation relative to the supportingrframe The second pick-olf 41 is coupled through the transformation device 31 and a second amplifier 4s to a motor 5| arranged to oppose any forces tending to disturb the orientation of frame Il about axis 25-2 5. it will be apparent that amplifiers 39 and 43 and motors 4| and 5| not only prevent disturbing forces from exerting prolonged effect upon the principal frame Il, but also act to retain gyroscopes I3 and l5 in their predetermined respective orientations relative to frame Il.

The transformation device 31 is a variable transformer arrangement (Fig. 2) having two primary windings 33 and 38 and two secondary windings 40 and 42. The primary windings 35 and 38, arranged for freedom from mutual coupling therebetween, may be mounted on a rotary member 44 connected to frame Il; while the secondary windings 4D and 42, similarly arranged as in quadrature positional relation for freedom from mutual coupling, are xed to the cooperating member 46. When the frame I I and Cardan suspension element I9 occupy the relative positions in which they are shown in Fig. 1, the primary winding 36 which is connected to pick-olf 3| is closely coupled to the secondary winding 40 which is connected to thel input circuit of amplifier 39; and windings 38 and 42 similarly provide maximum coupling between pick-off 41 and theV input circuit of amplier aca-Mss.

and element I9 are changed through a 90?* ex.r

tent, so that windings 36 and 38 are rotated 90' relative to windings 40 and t2, then the mutual couplingbetween windings 36 and 40 and between windings 38 and I2V isA eliminated, being supplanted by maximum mutual coupling between windings 3e and 42 and between windings 38 and de. Then pick-off 3j is coupled only tothe circuit including amplifier 49 and motor I., and; piola-offV 41 is coupled only to amplifier 39. and motor 4I. Such an interchange of' coupling relations of the pick-offs and the suspension motors is required to maintain proper coordination of the associated gyroscope axes and suspension motor axes, so that the suspension motor which` respondsy to an angular displacement of a gyroscope about its gimbal journal. axis is the motor positioned to apply the torque necessary to restore` that. gyroscope to its, neutral position.

For intermediate changes of relative positions of: frame II and suspension element I9, each primary winding is coupled to corresponding intermediate extents to both secondary windings 40; and s2, so that departureof ahorizontal-axis stabilization gyroscope from its neutral position results in the production of appropriate torque components in both horizontal-axis motors llly and 5I.

The transformation device 31 may be especially constructed for the apparatus illustrated, or it may be produced by combining two commercially available variable transformer devices having quadrature winding relations, as for example, by tandem coupling two Telegon units with 90 relative displacement of their rotors, the rotor windings being employed as the primaries and the corresponding stator windingsA being employed asl series,` secondaries, for example- In accordance with an important feature of the present invention, gyroscopev I3 is arranged with its center of gravity displaced from its normally vertical gimbal journal axis tothe side thereof characterized by downward peripheral movement of the rotor 21. This maybe accomplished, for example, by the provision of a weightA 3,0; supported on aA weight arml 32 rigidly fastened to the rotor-bearing frame 29 and extending` outwardly therefrom, or the assembly, in-

cluding rotor-bearing frame 29 and rotor 2I,V

II and suspension element I9 is vertical, the

gimbal journal axes of gyroscopes I3 and I5 are vertical and hence parallel withl the earths gravitational force eld, and accordingly, the eccentricity of their masses exerts no torque about their gimbal journal axes. In this condition, the mass exerts a torque about an axis perpendicular to the gimbal journal axis and parallel to the rotor spin axis. Upon inclination ofthe framer II and the gimbal journal axes of the, gyroscopes, however, the eccentric masses werent-,produce torquesaboutthe.simhalpioumal.

frame II. For slight inclination of the frame I I, the erection torque produced in this manner is substantially proportional to the angle of inclination.

The third gyroscope I1 is arranged to afford stiffness or rigidity to frame II about the vertical axis. This gyroscope I'I includes a rotor 69 supported for rotation about a horizontal axis in a rotor-bearing frameY 1I, which, in turn, is pivoted for freedom about a third gimbal journal axis in the frame II. Gyroscope I'I is similarly provided with a precession-responsive pick-off i3 coupled through an amplifier I5 to a verticalaxis torque motor 11. In this arrangement, gyroscope Il, while rigidly opposing forces tending to rotate frame II about the vertical axis, responds to these forces with precession about its horizontal gimbal journal axis, and as a result, motor 'I1 is-` actuated to oppose the disturbing forces, and to restore gyroscope, I1 to its neutral orientation in framev II.

The azimuth stabilization gyroscope Il may be retained in alignment with the horizontal component of the earths magnetic force field by the provision of one ormore vertical bar magnets 853 connected to rotor-bearing frame lI, as -by extension arms 82. As arranged, these magnets produce no torque upon gyroscope` Il about the gimbal journal axis thereof when the gimbal journal axis is aligned as indicated with the horizontal component of the earths magnetic field. Upon deviation of frame I I about the vertical axis from this orientation, however, the magnets S@ react withthe earths magnetic field to produce a couple about the gyroscope gimbal journal axis in the direction to cau-se procession thereof to restore alignment with the horizontal component of the magnetic force eld. Here, again, for small departures, the azimuth precessional torque exerted is substantially proportional to the angle of departure from alignment with the earths force field. Such arrangements are broadly referred to in the art as directional gyroscopes slaved to the magnetic meridian or simply as slave gyroscopes or gyro-magneticcompasses.

Remote positional transmitter units 81, 89 and el may be provided for supplyingcraft or` body orientation reference signals to utilization apparatus such as a gun fire control system or a craft attitude control system such as an aircraft auto-pilot.

With the arrangement of elementsv as illustrated in Fig. l, it will be apparent that even if suspension torque motors M 5I and TI were inoperative for a brief period of time, the gyroscopes I3, I5, and I'I would oppose anyV rapid changes of orientation ci the principal reference frame Il. However, disturbing torquesexertedl upon frame I I would cause precessions of the gyroscopes about their respective gimbal journal axes, and if prolonged'would eventually upset the desired mutual orientation relationships among they stabilization gyroscopes. This` is particularlytrue in instances where the transmitter units 8l, and 9i are required to be of such size and signal output capacity that they tend materially to interfere with the freedom of the gyroscope suspension system. With the torque motors operating, however, any disturbing tcrqueaiecting the principal frame I I toy such anY extent as to cause4 even minuteprecession of one of the gyroscopes supported therein not only is opposed immediately by the gyroscopic stiffness of thatl gyroscope, but also is. Opposedbr tha ngwerfulsusnension torque. mp-

tor coupled through an amplier to receive the signal provided by the precession pick-off unit associated with the gyroscope. The torque motor provides its opposing torque for the duration of the disturbing force, and sufficiently longer to cause procession of the gyroscope back to its neutral position. Consequently, net efiects of the disturbance both on the space orientation of frame l l and on the relative displacement between the frame and the disturbance-opposing gyroscope are substantially cancelled.

The gyroscope orientation reference system is rendered relatively free from vulnerability to reaction torques experienced with the positional transmitters 3'?, 89 and 9i. Moreover, the gyroscopes I3 and l5 which act to stabilize the frame il about the horizontal axes are arranged for freedom from error resulting from axial shifts in rotor cr gimbal bearings of the center of gravity of the rotor or rotor-bearing frame.

In this stable reference apparatus, the Cardan suspension torque motors 4i, 5i and 'il and their associated amplifiers 39, 49 and l5 need not meet any such exacting requirements as are ordinarily associated with servo systems requiring prompt and accurate positional follow-up action. With a readily provided torque system, the gyroscopes i3, it and l? are prevented from precession farther` than through an angular displacement of the order of a degree, with a prolonged rotation of the craft about any axis, and accordingly, the principal reference frame l! may be compressed into a layout of extreme compactness. For example, the horizontal diameters oi the rotors 2, d3 and |59 may be arranged in a configuration corresponding to the legs and hypotenuse, respectively, of a horizontal isosceles right triangle.

With the present invention, supervisory orientation control over a gyroscopic stable reference system in accordance with the earths force elds is accomplished without the requirement or any relative movement of erector parts or azimuth prccessor parts. The rotor-bearing ira-ine of each gyroscope rigidly incorporates the force iield-re sponsive element, so arranged that upon deviation of the stable reference system, a corrective torque is exerted in quadrature relation to the direction of movement required for restoration to the desired orientation, and is correctly aligned for causing vsuch precession of the appropriate gi rcscope as required for directly overcoming the deviation.

Throughout the drawing, the electrical circuit connections have been schematically indicated as extending directly between the electrically interconnected parts of the apparatus. In practice, however, slip rings and brushes are provided at the journals of intermediate suspension elements, and the conductors are arranged in cable sections extending around the Cardan suspension elements between successive journals. Such details of circuit arrangements are well-known, as are also the provisions of pneumatic or fluid -drive and fluid torque systems which may be employed if desired.

Since many changes could be made in the above construction and many apparently widely diierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. Stable reference apparatus comprising: a

movable principal frame to be stabilized about a vertical axis; a gyroscope for detecting and resisting forces tending to disturb the orientation of said principal frame about said vertical axis; said gyroscope comprising a rotor-bearing frame journaled in said principal frame for freedom about a gimbal journal axis transverse said vertical axis, and a rotor supported therein for rotation about a rotor spin axis transverse said gimbal journal axis and normally transverse said vertical axis; and means fixed to said rotorbearing frame for providing a magnetic field aligned transverse said gimbal journal axis and ixedly in the `plane of rotation of said rotor for interaction with the earths magnetic eld to exert a torque on said gyroscope about said gimbal journal axis for eiecting procession of said gyroscope about said vertical axis, whereby variations of the azimuth orientation of said principal frame from alignment with the earths magnetic field are suppressed by said precession.

2. Stable reference apparatus as dened in claim 1,. said principal frame being pivotally supported for freedom about a normally vertical major axis, and said apparatus further including gyroscope means :for retaining said major axis vertical.

5'. Stable reference apparatus comprising: a principal frame to be stabilized in space about Erst and second mutually perpendicularmajor axes and for the establishment of a third major axis perpendicular to said rst and second major axes; a first gyroscope coupled to said frame for detecting and resisting forces tending to disturb the space orientation of said principal frame about said first major axis, said rst gyroscope comprising a rst rotor-bearing frame journaled' in said principal frame for rotation about a rst gimbal journal axis perpendicular to a line extending .parallel to said rst major axis, and a first rotor supported in said iirst rotor-bearing frame for rotation therein about an axis perpendicular to af line extending parallel to said first gimbal journal axis and normally perpendicular' to a line extending parallel to said rst major axis; a second gyroscope coupled to said frame for detecting and resisting forces tending to disturb the space orientation of said principal frame about said second major axis, said second gyroscope comprising a second rotor-bearing frame journaled in said principal frame for rotation about a second gimbal journal axis lperpendicular to a line extending parallel to said second major axis, and a second rotor supported in said second rotor-bearing frame for rotation therein about an axis perpendicular to a line extending parallel to said second gimbal journal axis and normally perpendicular to a line extending parallel to said second major axis; and means responsive to deviation of said principal frame about said rst major axis from a predetermined orientation about said rst major axis for applying a torque to said rst rotorbearing frame about said rst gimbal journal axis to produce procession of said rst gyroscope suppressing said deviation, means comprising means providing an eccentric position of the center of gravity of said rst gyroscope from said first gimbal axis.

ll. Stable reference apparatus comprising: a principal frame to be stabilized in space about rst and second mutually perpendicular major axes and for the establishment of a third major axis perpendicular to said first and second major axes; a iirst gyroscope coupled to said frame for said last-named' detecting fand `resistn'g forcestending to disturb the #space orientation of vsaid principal frame about naidf-rst major axis, `said 'first lgyros'cope comprising fa iirst rotor-bearingsframe journaled 'insaidprincipal frame for rotation therein about Yla 'iirst gimbal journal axis perpendicular to a lline extending ,parallel to said dirsi'. major axis, 'and a first rotor supported A'in `said 'r'st r'o'torbearing frame for rotation Ytherein about an axis i'perpendic'ul-ar to aline extending 'parallel to fsa'id rtir-st gim'bal journal axis and f'n'ormally perpendicular to a line extending parallel to said rst major axis; and va'secondgyroscpe coupled to said A'frame ufor detecting 'and resisting vforces 'tending to luist-urb the vspecie errements for said l principal frame about said second major axis, said second gyroscope comprising a second rotorbearing frame journaled in said principal frame for rotation therein about a second gimbal journal axis perpendicular to a line extending parallel to said second major axis, and a second rotor supported in said second rotor-bearing frame for rotation therein about an axis perpendicular to a line extending parallel to said second gimbal journal axis and normally perpendicular to a line extending parallel to said second major axis; said second gimbal journal axis being normally vertical and said second rotor axis being normally horizontal, and the center of gravity of said second gyroscope being displaced from the plane passing through said second gimbal journal axis parallel to said secand rotor axis in the direction of downward peripheral movement of said second rotor.

5. Stable reference apparatus comprising: a principal frame to be stabilized in space about rslt and second mutually perpendicular major axes and for the establishment of a third major axis perpendicular to said rst and second major axes; a rst gyroscope coupled to said frame for detecting and resisting forces tending to disturb the space orientation of said principal frame about said i'lrslt miajor axis, said rst gyroscope comprising a rst rotor-bearing frame journaled in said principal frame for rotation about a iirst gimbal journal axis perpendicular to a line extending parallel to said rst major axis, and a first rotor supported in said first rotor-bearing frame for rotation therein about an axis perpendicular to a line extending parallel to said first giimbal journal axis and normally perpendicular to a line extending parallel to said major axis; and a second gyroscope coupled to said frame for detecting and resisting forces tend-ing to disturb the space orientation of said principal frame and about said second major axis, said second gyroscope comprising a second rotor-bearing frame journaled lin sa-id principal frame for rotation about a second gimbal journal axis perpendicular to a line extending parallel to said second major axis, and a second rotor supported in said second rotor-bearing frame for rotation therein about :an axis perpendicular to a line extending parallel to said second gimbal journal axis and normally perpendicular to a line extending parallel to said second major axis; said iirst and second gimlbal ,journal axes being normally vertical and said firstV and second rotor axes being norm-ally horizontal, and the center of gravity of each of said gyroscopes being displaced from the plane passing through the respective gimbal journal axis parallel to the respective rotor axis in the direction of Vdownward peripheral movement of the respective rotor.

6. Stable reference apparatus comprising.: a

isps-nist ff'rar'nve tok be 'stabiilifzi-*Ld. in 5spafce rvabout l'rst and second horizontal major 'axesand vertical axis: rst and second gyroscopesjcoupled to said trame for detecting #and lresisting forces tending to Vdisturb the space orientation -of said principal frame aboutJ said y-rlrst and second horizontal major axes, respectively, said iirst and second gyroscopes respectively vcompri-sing first and second "rotor-bearing frames j'ournalled in said principal frame for 'freedom about vertical gimba-l journal axes therein and rst and "second rotors 'respectively 'supported Vin v@said froto'r'- bearing tramesror rotation therein about ii-rst and second horizontal spin axes respectively perpendicular tb said first and second horizontal r-najor 'axes; a third 2gyrosf'zope Acoi'lpled to VAsaid frame :for detecting and resisting fo'rce's 4"teiid'irfg to disturb the orientation of said principal frame about said vertical axis, said third gyroscope comprising a third rotor-bearing frame journalled in said principal frame for freedom about a horizontal gimbal journal axis, and a third rotor borne in said third rotor-bearing frame for rotation about a horizontal axis transverse said horizontal gimbal journal axis; said first and second gyroscopes having their centers of gravity displaced from planes through their respective gim-bal journal axes parallel to their rotor spin axes in the respective directions of downward peripheral movement of the rotors.

7. Stable reference apparatus as deiined in claim 6, further including means coupled to said third rotor-bearing frame for producing a unidirectional magnetic field in predetermined directional relation therewith for reacting with an ambient magnetic eld, whereby the direction of said principal frame is controlled according to said ambient magnetic eld.

8. A gyro-magnetic compass comprising: a vertical ring, a gyro rotor bearing frame journalled therein about a normally horizontal axis and a magnet fixed to the said frame in a normally substantially vertical position, said magnet cooperating with the earth"s magnetic eld to apply a torque round said axis depending on deviation of the gyro from the magnetic meridian so as to cause the gyro to process in a direction to eliminate said deviation.

9. A gyro-magnetic compass comprising: a principal frame rotatably mounted about a normally Vertical axis, a rotor having its plane of spin normally in the earths magnetic meridian and a rotor bearing frame journal-led in said principal frame about a normally horizontal axis, said frames and rotor constituting a free gyro, and a magnet iixed to said rotor bearing frame with its axis substantially Vertical so that the earths magnetic field causes said magnet to exert a torque round the rotor bearing frame axis depending on the deviation of the gyro from its normal orientation and causing the gyro to precess about said vertical axis in a direction to eliminate said deviation.

1G. Stable reference apparatus for use in airplanes or other movable craft comprising: a principal frame journalled on the craft for freedom in azimuth; a gyroscope in said frame for detecting and resisting forces tending to distur-b the orientation of said principal frame in azimuth; said gyroscope comprising a rotor frame journalled in said principal frame for freedom about a gim-bal axis which is normally horizontal, and a rotor journalled in .said rotor frame for spinning therein about a normally horizontal axis transverse to said gimbal axis;

a bar magnet xed to said rotor frame with its polar axis perpendicular to the gimbal axis, the field of said magnet reacting with the earths magnetic eld upon deviation of said gimbal axis from the magnetic meridian to exert a directtorque on said rotor frame to produce precession of said gyroscope in a direction to restore the gimbal axis to the meridian.

11. Stable reference apparatus as claimed in claim 10, in which said bar magnet is normally vertical and is assisted by a second bar magnet spaced therefrom but similarly disposed.

WILLIS G. WING.

REFERENCES CITED The following references are of record in the le of this patent:

Number Number 12 UNITED STATES PATENTS Name Date Boykow July 14, 1925 Boykow Apr. 21, 1931 Koster Jan. 30, 1940 Roters Sept. 7, 1943 Carter Jan. 23, 1945 Bates Aug. 7, 1945 Davenport Apr. 13, 1948 vBousky July 20, 1943 Hull Apr. 26, 1949 FOREIGN PATENTS Country Date France Nov. 18, 1938 

